xen: Fix Kconfig dependencies

XEN config can be enabled by other platforms (even it doesn't need to make
sense) that's why fix dependencies. XEN (xenbus.c) requires sscanf (also
pvblock needs it). And PVBLOCK is inside drivers/xen folder which requires
XEN to be enabled.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>